Try clicking on the "Reduce" button:

Notes: you have to fully specify parentheses and separate your lambdas. So for example, you can't write `(\x y. y)`; you have to write `(\x (\y y))`.

The parser treats symbols that haven't yet been bound (as `yes` and `no` in the example above) as free variables.

If you try to evaluate a non-terminating form, like `((\x (x x)) (\x (x x)))`, you'll probably have to force-quit your browser and start over. Anything you had earlier typed in the upper box will probably be lost.